1. Field of the Invention
The invention relates to drilling fluids used in the drilling of subterranean oil and gas wells. In particular, the invention provides a water-soluble synthetic component which provides lubricity and shale inhibiting properties comparable to those of an oil-based drilling fluid without the adverse effects thereof.
2. Brief Description of the Prior Art
A rotary system is the most common form of drilling a subterranean well. This system depends upon the rotation of a column of drill pipe to the bottom of which is attached a multi-pronged drilling bit. The drill bit cuts into the earth, causing the cuttings to accumulate as drilling continues. As a result, a drilling fluid must be used to carry these cuttings to the surface for removal, thus allowing the bit to continue functioning, and the bottom of the hole to be kept clean and free of cuttings at all times
Although aqueous-based drilling fluids which utilize clear water, brine, or sea water as the primary liquid phase may be found to be dominant within some facets of the drilling industry, drilling fluids have been developed and commercialized utilizing a hydrocarbon liquid as the major liquid constituent. The use of such a hydrocarbon based fluid or water-in-oil emulsion fluid is quite common in which the drilling fluid is utilized in the higher temperature wells which are drilled to the more substantial depths. These hydrocarbon fluids are referred to as oil based fluids and normally exhibit very desirable fluid flow characteristics within their respective temperature utilization ranges. Additionally, such fluids are recognized as having enhanced lubricity characteristics, for the purpose of lubricating the drilling bit and preventing differential sticking of the drill pipe. In addition, some of such fluids have exhibited considerable beneficial shale stabilization properties to prevent erosion of shale or clayey substances in the well which soften on hydration when exposed to an aqueous fluid formulation. Such hydrocarbon based fluids are known also to exhibit excellent filtration properties by incorporation of a filtration control agent into the composition to inhibit filtrate loss to the formation around the bore hole.
While such hydrocarbon based fluids have been widely accepted in the industry and have considerable advantages, such fluids also have disadvantages, including environmental pollution effects resulting from, for example, the hydrocarbon fluid being accidentally discharged into the sea or ocean when used on offshore wells. There is also some difficulty in preparation at offshore locations where the barging of considerable quantities of hydrocarbon based fluids is cumbersome and comparatively expensive. Currently all oil-based fluids used offsore U.S.A. and cuttings contaminated with oil-base fluid must be contained and hauled back to shore. This is, obviously, an expensive operation.
Shale stabilization in a fluid suspension is believed to at least partially be dependent upon inhibition of swelling and dispersion of the shale into the fluid. By "shales" and "shale" is meant to refer to materials such as bentonite and the like, claystones and "gumbo" -type colloidal-clay substances and related substances which possess the property of hydrody namic volume increase when exposed to aqueous environments. Of particular importance is the recent geological sediment referred to as "gumbo" encountered in the drilling of some subterranean wells. This "gumbo" contains a high percentage of smectite, hydrates in water rather rapidly, becomes soft and deformable, and may swell to many times its original size. By "swelling" we mean to refer to the hydrodynamic volume increase of the shale. By "inhibit", "inhibition" and "inhibition of swelling" we mean the ability of a process to retard the hydration of shales whereby they remain intact and basically in their original size, shape and volume, said process being the preparation of an inhibitive fluid suspension and introducing the drill cuttings generated by the drill bit into the drilling fluid suspension as it is circulated through and out of the well.
Swelling of shales is believed to be attributable to at least two mechanisms: crystalline and osmotic. Crystalline swelling also known as surface hydration results from the adsorption of mono-molecular layers of water onto the shale surfaces. Shales usually contain a mineral component which has an expandable layer-type structure which allows water to be adsorbed on these inter-layer surfaces thus causing additional swelling of the shale. Osmotic swelling occurs if the cation concentration on the surfaces of the shale is greater than that in the surrounding fluid. This difference draws the water phase between the layers whereby ion hydration and water adsorption occurs causing an increase in the hydrodynamic volume.
For swelling to occur by either of the aforementioned processes, the shale must interact with the water taking and/or sharing hydroxyl groups of the water with it. In present drilling operations, shale inhibition is achieved by addition of divalent cations or potassium ions through base exchange or by the addition of encapsulating and bridging polymers to the water-based fluids or, by the use of water-in-oil emulsion fluids (oil based fluids).
We have discovered a drilling fluid which provides beneficial shale and borehole stabilization without the addition of salts and/or polymers or the use of the water-in-oil emulsion fluids. With this fluid, the reactivity of the shale is partially satisfied by a hydroxyl-bearing water-soluble synthetic component in the fluid phase. The preferential adsorption of these water-soluble additives fulfills the surface reactivity requirements of the shale without the formation of bonds between particles and reduces the adsorption of the multi-layers of water which can cause swelling.
The fluid is comprised of the following: (1) a liquid phase containing (a) a water phase comprising fresh water, seawater, brine, simulated brine, or mixtures thereof; and (b) a water-soluble component selected from the class consisting of polyhydric alcohols, glycol, glycol ethers, polypropylene glycols, polyethylene glycols, ethylene oxide-propylene oxide copolymers ("EO-PO"), alcohol-initiated EO-PO copolymers and/or mixtures thereof, the ratio of said water-soluble component in the total liquid phase being from about 5% to about 50% by volume; (2) a viscosifier for suspension of solids in said aqueous phase; and (3) a filtration control agent. The fluid with the water soluble component will exhibit a lubricity coefficient lower than that for substantially the same fluid without the water soluble component as determined by the American Petroleum Institute's "Procedure for Determination of Lubricity Coefficient (Tentative)" (1980), and the linear swelling on a reconstituted "gumbo" shale inserted for about 60 minutes in said drilling fluid being lower than that for substantially the same fluid without the water soluble component, as measured by the "Swelling Test", "Rigsite Shale Evaluation Techniques for Control of Shale-related Wellbore Instability Problems", SPE/IADC Paper No. 16054, pages 52-53, (1987).
While particular water soluble synthetic additives have been used in the petroleum industry in the past, their use as a fluid phase component which enhances the lubricating and shale inhibiting characteristics of a drilling fluid has not been appreciated by those skilled in the art. Typical of such prior art is U.S. Pat. No. 4,498,994, which teaches the use of several polyhydric alcohols or other materials as a solvent for salt, such as calcium bromide brine, in a solids free completion fluid and method. Chemicals such as polypropylene glycols are used as industrial lubricants and hydraulic fluids in other industries.
The present invention comprises a drilling fluid having a liquid phase which imparts lubricity and shale swelling inhibition properties to the fluid similar to the beneficial properties of oil-based drilling fluids without the accompanying adverse effects, such as high toxicity and the like.